AVS 67 Session 2D-Contributed On Demand: 2D Materials Contributed On Demand Session

In recent years, several transition metal dichalcogenides (TMDs) have been found to exhibit intrinsic ferromagnetic properties near the monolayer limit [1,2]. However, manganese diselenide (MnSe₂) is of special interest among them since its monolayer 1T polytype has been found to display long-range magnetic ordering and high magnetic moments near room temperature conditions, making it uniquely qualified for various spintronic applications [3,4]. In this study, two-dimensional heterostructure of MnSe₂ is grown via Chemical Vapor Deposition (CVD) on few layers epitaxial graphene (EG) synthesized on 6H silicon carbide (SiC) substrate. Two different CVD growth approaches have been investigated in this study: Thermal Vapor Deposition (TVD) and Thermal Vapor Selenification (TVS). In the TVD approach, selenium (Se) powder precursor was used alongside either manganese (IV) oxide (MnO₂) or manganese acetate (Mn(CH₃CO₂)₂) powder for direct CVD growth of MnSe₂ on epitaxial graphene. In the TVS approach, δ-phase MnO₂ is first electrodeposited on EG using a manganese acetate solution, which is then selenified via CVD using only Se powder precursor.

The samples grown were characterized via Raman, SEM, and AFM tools, all of which demonstrated the presence of MnSe₂ growth for both TVD and TVS mechanisms. For both precursor combinations of the TVD approach, the characteristic A_g Raman peak of MnSe₂ was visible at 269 cm^-1, although its low intensity (almost 40-50% of the control FTA peak of the substrate at 203cm^-1) indicates small grain sizes and yield density, a conclusion supported by the SEM and AFM images (around 0.1-0.3μm diameter grains). The yield was found to be slightly higher (about 300% larger grain size) for MnAc compared to MnO₂. For the TVS approach, however, both E_g and A_g peaks were found to be visible (at 145cm^-1 and 267cm^-1 respectively) with extremely high peak intensity (around 500% of the control peak), implying significantly higher MnSe₂ yield, as also evident in the AFM and SEM data characterized by a high deposition density along the graphene step edges. This comparative study of the two approaches clearly shows TVS to be the far superior mechanism compared to TVD for MnSe₂ growth on epitaxial graphene. The TVS approach, once optimized, should result in single-crystal monolayer growth of MnSe₂ heterolayer on the epitaxial graphene substrate, potentially leading to the wafer-scale synthesis of MnSe₂ based spintronic devices.

Reference:

[1]Y. Ma et al, ACS Nano, 2012, 6, 1695–1701

[2]Y. Zhou et al, ACS Nano, 2012, 6, 9727–9736

[3]M. Kan et al, Phys. Chem. Chem. Phys. 16 (2014) 4990

[4]D. O’Hara et al, Nano Letters 2018 18 (5), 3125-3131

In recent years, a van der Waals heterostructure device, stacking 2D architectures atomically with synergistic combinations of nanomaterials have attracted tremendous attention with many potential applications such as chemical sensors in environmental and safety monitoring, and medical diagnostics and biomedical health care systems [1,2]. Atomically thin 2D graphene and transition metal dichalcogenides (TMDs) have an extremely high surface-to-volume ratio which is the most critical parameter for chemical sensing applications. Here we have fabricated a heterostructure of molybdenum disulfide (MoS₂) nanoflower and epitaxial graphene on 6H silicon carbide (SiC) substrate for chemical sensing. We have combined the advantages of high sensitivity and fast response time of graphene with the high surface-to-volume ratio of MoS₂ nanoflower to develop a room temperature chemical sensor for health/environmental monitoring systems.

Bilayer epitaxial graphene (EG) was synthesized by Si sublimation on 6H SiC, and MoS₂nanoflowers were grown on graphene/SiC directly using metal-organic chemical vapor deposition (MOCVD). The growth methods of graphene and MoS₂ nanoflowers are discussed in detail elsewhere [3,4]. The structural and optical properties of the samples were investigated by scanning electron microscopy (SEM), atomic force microscope (AFM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, photoluminescence (PL), and cyclic voltammetry (CV). To fabricate the chemical sensor, e-beam lithography and reactive ion etching (RIE, CF₄) were used to prepare a simple pattern. Finally, the metal electrode (Ti/Au = 30/120 nm) was deposited using an e-beam evaporator. The prepared chemical sensor was tested with various gases such as 5 ppm of nitrogen dioxide (NO₂) and ammonia (NH₃), and ~1000 ppm of volatile organic compounds (VOCs) and showed a superior chemical response and recovery at room temperature. It is evident that the prepared device is suitable for chemical sensing applications such as health and environmental monitoring systems.

References: [1] W. Yang et. al. Inorg. Chem. Front. 3, 433(2016); [2] X. Zhou et. al. Chem. Rev. 115, 7944 (2015); [3] B. K. Daas et. al. J. Appl. Phys. 110, 113114 (2011); [4] J. Choi et. al. Nano Lett. 17, 1756 (2017).

The quantum Hall (QH) effect, a topologically non-trivial quantum phase has brought into focus the concept of topological order in physics. The topologically protected quantum Hall edge states are the essential features of the QH effect, however microscopic local probe studies of edge state have been limited. The QH edge states in graphene are special since they emerge from four-fold nearly-degenerate Landau levels. In this talk, we present a microscopic study a of the QH edge states originating from the broken symmetry zero Landau level (zLL) in graphene. The edge states emerging from integer filling factors of ν = 0, ±1 are spatially mapped across the quantum Hall edge boundary using atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) technique. The measurements of chemical potential within the four-fold degenerate zLL suggest a change in the microscopic spin/valley ordering as a function of magnetic field caused by the interplay of the moiré superlattice potential of the graphene/hBN system and other symmetry breaking effects.

Transition metal dichalcogenides (TMDs) ternary alloys have composition dependent electronic properties, such as bandgap, work function, and electrical response (n-type or p-type conductivity). When individual ternary alloys are combined to form heterostructures, their functionality can be greatly expanded compared to heterostructures based on binary alloys. Thus, opening a path for new optoelectrical applications with flexible designs and enhanced properties. In this work, we successfully synthesize lateral heterostructures with monolayer, bilayer and trilayer thicknesses based on ternary alloys of the type MoSe_2(1-x)S_2x-WSe_2(1-x)S_2x, where x is the chalcogen atom composition. The continuous tuning of the band gap for both materials, is demonstrated. The composition homogeneity of each domain was studied by photoluminescence (PL) and Raman spectroscopy, while Kelvin probe force microscopy (KPFM) was used to study the tunable band alignment at the heterojunctions.

TiS₃ and ZrS₃ are quasi-one-dimensional n-type van der Waals semiconductors that offer an opportunity to fabricate transistors with widths < 10 nm, as they lack undesirable edge effects unlike other well-studied two-dimensional materials (namely: graphene and transition metal dichalcogenides). TiS₃ and ZrS₃ have modest band gaps of ~1 eV and ~1.8–2.1 eV, respectively, and both the materials are highly anisotropic. The combination of adequate band gaps and appreciable electron mobilities indicates some potential for these metal trichalcogenides materials in applications in beyond CMOS sensor electronics. Temperature dependent x-ray photoemission spectroscopy (XPS) measurements were employed to obtain a quantitative picture of electron-phonon scattering in these materials. The Debye–Waller factor plots for S 2p core-level of TiS₃, based on the temperature dependent XPS measurements, hint that effective Debye temperatures of the two different kinds of sulfur coordination are 464 ± 35 K and 547 ± 26 K. Whereas the Debye–Waller factor plots for the S 2p core-level of ZrS₃ suggest effective Debye temperatures of 558 ± 30 K and 667 ± 35 K for, again, the two different sulfur coordination. The apparent difference between the effective Debye temperatures of different sulfurs, within the material, is an expected consequence of dissimilar coordination number. Additionally, the high effective Debye temperatures of the different sulfur species in ZrS₃ than in TiS₃ imply that the trichalcogenide of zirconium is stiffer than its titanium counterpart, which translates to reduced electron-phonon scattering in the former. This conclusion is further supported by the temperature dependent filed-effect mobility measurements, which consistently indicate that the carrier mobility of ZrS₃ is less sensitive to temperature changes in comparison with that of TiS₃. Even though the electron-phonon scattering is implicated as the cause for the much lower measured mobilities compared to the predicted high carrier mobilities (~10,000 cm²V^–1s^–1 for TiS₃ and ~1,800–2,500 cm²V^–1s^–1 for ZrS₃), the relatively high Debye temperatures indicate that there are only a limited number of soft phonon modes for these materials, which can be possibly suppressed, leading to improved mobilities.

Angle-Resolved Photoemission Spectroscopy (ARPES) is a fundamental tool to visualize the momentum-dependent electronic bands of materials, revealing relevant characteristics, parameters and new interactions [1, 2] in them. Spin-ARPES (SR-ARPES) adds up spin resolution and, by
measuring spin polarization, allows the complete determination of a quantum state [3]. The SRARPES technique has been proven to be extremely powerful in recent years as it allows studying spin-polarized electronic states in two-dimensional (2D) systems with strong spin-orbit coupling
(SOC), whose related phenomena, such as the Rashba effect or the appearance of topological insulators (TI), give rise to new interactions and particularly relevant electronic states, which can be exploited in spintronics and spin-orbitronics [4]. In this sense, interfacing Rashba or topological
surface electronic states with ferromagnetic thin films (FM/SOC) [5, 6] can modify their spin texture and be at the origin of emerging phenomena as spin-charge conversion, switching out-of-plane magnetization by spin transfer-torque, DM Interaction and chiral spin structures as skyrimions, or the
quantum anomalous Hall Effect [7]. Thus, controlling interfacial interactions and their effect on spin-polarized electronic states is crucial. In this contribution we will present the new SR-ARPES facility at IMDEA Nanoscience which includes a 3D mini-Mott Spin Detector, which allows the determination of the 3D spin components (two in-plane and one out-of-plane). So far, we have measured the Rashba surface state spin texture in Au(111) (Fig. 1), including Fermi surface (FS) mapping and spin resolved measurements in valence band (VB) and surface state (SS). Finally, some perspectives for the study of 2D topological quantum materials or emerging phenomena in FM/SOC interfaces will be presented as well as some experiments related to the abovementioned phenomena arising from SOC and that, in the light of previous results obtained in the group, are expected to be continued with this new system and further developed in
different synchrotron facilities.

References
[1] Lu, D. et al. (2012). Annu. Rev. Condens. Matter Phys., 3(1), 129-167.
[2] Yang, H. et al. (2018). Nature Reviews Materials, 3(9), 341-353.
[3] Okuda, T. (2017). Journal of Physics: Condensed Matter, 29(48), 483001.
[4] Soumyanarayanan, et al. (2016). Nature, 539(7630), 509-517.
[5] C. Carbone et al., Phys. Rev. B 93 125409 (2016).
[6] J. C. Sánchez-Rojas et al., Phys. Rev. Lett. 116, 096602 (2016)
[7] Chang CZ, et al. Science. 2013 340 167-70.
[8] Chang, C., et al. Nature Materials, 14, 473–477. 2015.

Two-dimensional (2D) materials with various electrical and optical properties are attracting large research interests. Moreover, ferromagnetic(FM) 2D materials possess interesting possible applications such as sensors and data storage. For example, CrI₃ has been shown to exhibit FM properties as either bulk or monolayer structures. However, its relatively low Curie temperature of ca. 45K hinders device application of CrI₃. In this work, we employ density functional theory (DFT) calculations with spin orbit coupling (SOC) to examine the adsorption of NH₃, PH₃, NO and NO₂ on 2D monolayer CrI₃. The variations in the adsorption energy and charge transfer, and the resulting changes in the band structure and the magnetic anisotropy are analyzed. In addition, the shifts in the Curie temperature according to the molecular adsorption are estimated via Monte Carlo simulations assuming Ising model.

Graphene microstructures have been shown to exhibit controllable directional electron emission when carrying a current under an accelerating electric field. Through Phonon-Assisted Electron Emission (PAEE), graphene microstructures demonstrate electron emission at electric field intensities and lattice temperatures below what would be expected for carbon to demonstrate field emission or thermionic emission, respectively. Furthermore, such emissions tend to be out of the plane of the graphene microstructure, allowing control of the directionality of the emission current even before being directed with an applied electric field.

Thus far, such arrangements have tended to involve structural dimensions in the range of micrometers, commonly with CVD-transferred graphene and carbon nanotubes. However, even with hundreds of kV/cm applied fields, emission currents rarely exceed a few nA. Herein are examples of the characteristics and tunable variables demonstrated by multiple epitaxial graphene structures, with dimensions up to cm, capable of producing >1 μA emission currents in applied fields of 2 kV/cm.

Quasi-freestanding epitaxial graphene on a silicon carbide substrate offers several advantages compared to transferred graphene or unzipped nanotubes, including lower defect density, greater structural integrity, ease of handling, and compatibility with simple photolithography fabrication techniques. The devices presented here were fabricated with a low-cost, wafer scalable, single-photomask process that obviates the need for material transfer to a second substrate and is easily tailored to create any variation in design parameters.

We have been studying various processes of energy dissipation in order to understand the nanoscale surface dynamics of Dirac materials experimentally and theoretically. Among this material class, topological insulators such as Bi₂Te₃ exhibit an insulating gap in the bulk while the surface is electrically conducting [1]. However, in real samples and at finite temperatures, their ideal zero-Kelvin behaviour is perturbed and scattering processes via electron-phonon (e-ph) coupling can give rise to energy losses. In this context atom-surface scattering has been demonstrated to be a sensitive probe to determine the surface phonon dispersion and the e-ph interaction parameter [1-3]. We will discuss the influence of the dimensionality on the e-ph coupling [4], when considering low-dimensional or quasi one-dimensional systems [5].

Furthermore, the lineshape broadening upon inelastic scattering from surfaces can be used to determine the characteristics of energy dissipation upon the motion of atoms and molecules [6,7]. Using this technique, we have studied the diffusion of water on the surfaces of Dirac materials (graphene and the topological insulator Bi₂Te₃)[7].Thus, we can specify the mechanisms underlying the motion of water and, by comparison with first-principle calculations, we identify aspects of its adsorption geometry, as well as the energy landscape for the motion. We see clear evidence for repulsive interactions between water molecules, which is contrary to the expectation that attractive interactions dominate the behaviour and aggregation of water. We are also able to make a qualitative assessment of the rates of energy transfer between water molecules and the topological insulator on which they move.[7].

Finally, we will discuss some recent experimental findings about the growth of hexagonal boron nitride (hBN) on Ru(0001). The growth of hBN on metal substrates, based on chemical vapour deposition, is well documented in literature [8]. In contrast to the reported structure of hBN after following the“ideal” growth conditions, we observe several intermediate structures, prior/parallel to the hBN growth, which, depending on the experimental conditions, can be transferred into each other or hBN.

[1] A. Tamtögl et al., Phys. Rev. B.95, 195401 (2017).

[2] A. Tamtögl et al., Nanoscale, 10, 14627 (2018).

[3] A. Tamtögl et al., npj Quantum Mater. 4, 28 (2019).

[4] G. Benedek et al., J. Phys. Chem. Lett., 11, 1972 (2020).

[5] P. Hofmann et al., Phys. Rev. B 99, 035438 (2019).

[6] A. Tamtögl et al., Carbon 126, 23 (2018).

[7]A. Tamtöglet al., Nat. commun., 11, 278 (2020).

[8] W. Auwärter, Surf. Sci. Rep, 74, 1 (2019).

Epitaxial FeSe thin films provide an ideal platform to probe the interplay of superconductivity and nematicity, due to the absence of long-range magnetic order. Here, we systematically investigate the nematic order in high quality bilayer FeSe/SrTiO₃ films grown by molecular beam epitaxy.Using low temperature scanning tunneling microscopy/spectroscopy, we observe features associated with Se atoms to be elongated along the Fe-Fe lattice direction within a specific energy window, demonstrating symmetry breaking from fourfold to twofold. Detailed analysis of Fourier transformation of the STM images reveals that the intensity of Fe Bragg peak breaks rotational symmetry within each Fe unit cell, indicative of an intra-unit-cell nematicity.Our results provide critical information on nematicity in Fe-based superconductors, an essential element in understanding superconducting transition in these materials.

This research is supported by DOE (DE-SC0017632).

Molybdenum disulfide (MoS₂) is one of the transition metal dichalcogenides (TMDs), having a layered structure like graphene. The monolayer of MoS₂ is an atomically thin semiconductor with a bandgap of 1.8 eV, which makes it good potential applications in nanoelectronics, optoelectronics, and sensors. Unlike graphene, which takes a significant portion in the studies about two-dimensional (2D) material, MoS₂ has many crystalline structures. Nanotribological study about the phase dependence will provide an important fundamental understanding of MoS₂, as well as TMDs.

In this contribution, we turned mechanically exfoliated 2H-MoS₂ into 1T-MoS₂ by lithiation process. We measured atomic-scale tribological and electrical properties of MoS₂ including friction, with atomic force microscopy (AFM). We report that with the phase transition of 2H to 1T, the friction of MoS₂ monolayer has greatly increased. We show that friction proportion of 2H-MoS₂ and 1T-MoS₂ is 0.19 : 1.1, with normalization to the value of mica. We attribute the friction increase of 1T-MoS₂ with increased overlap of phonon density of states (DOS) with mica substrate.

We also measured friction of MoTe₂, of which the pure 1T phase is commercially available since it shows stable 1T phase, as well as 2H phase. Friction proportion of exfoliated 1T-MoTe₂ and 2H-MoTe₂ is 0.12 : 1.07, normalized with the friction of mica. This study gives insight to the universal trend of the phase-dependent tribological effects on two-dimensional atomic layers.

In the weak-coupling Peierls' view, charge density wave (CDW) transitions are metal-insulator transitions, creating a gap at the Fermi level. However, with strong electron-phonon coupling, theoretically the effects of the periodic lattice distortion could be spread throughout the electronic structure and give rise to CDW gaps away from the Fermi level. Here, using scanning tunneling microscopy and spectroscopy, we present experimental evidence of a full CDW gap residing in the unoccupied states of monolayer VS2. Our ab initio calculations show anharmonic coupling of transverse and longitudinal phonons to be essential for the formation of the CDW and the full gap above the Fermi level. The CDW induces a Lifshitz transition, i.e., a topological metal-metal instead of a Peierls metal-insulator transition. Additionally, x-ray magnetic circular dichroism reveals the absence of net magnetization in this phase, pointing to a coupled CDW-antiferromagnetic ground state.

The fabrication of transistors which make use of the unique and versatile properties of transition metal dichalcogenides requires development of enabling technology for integration. This includes contacts, packaging, and stacking of electronically functional components. Ti-Au layers are common in transistor design and function as adhesion layer (Ti) and contact (Au). In our work the WS₂ flakes are transferred onto the Au-surface using micromechanical cleavage with a common adhesive, and subsequently integrated in a device structure. We probed the thermal stability of the Ti-Au-WS₂ stack by combining in-situ annealing with XPS analysis with small temperature intervals of 10-30°C in the temperature range from 200° to 625°C. The WS₂ layer itself is surprisingly stable with the onset of S-loss around 350°C, which is followed by formation of W-carbide and oxide (reaction with residue), and reduction to W(0) above 550°C. At the same time the Au-contact undergoes a dramatic change, and alloying with Ti occurs through rapid diffusive processes starting at 250°C, and concomitant reaction of surface Ti to form Ti-carbide and oxides through reaction with the residue from the TMD transfer. Ti-S compounds emerge only above 500°C. Alloying is suppressed if the Ti can rapidly react with surface carbon, oxygen and Ti diffusion is therefore transient limiting the formation of an alloy. However, if the concentration of the residue is low or a high coverage with WS₂ is achieved, the Ti-carbide and oxide formation is limited and Au-Ti alloys dominate. This reaction sequence leads to modification of the contact and can contribute to erratic, or irreproducible device performance. We will discuss the reaction pathways in detail, and use samples with and without WS₂, which acts as a control, and for different annealing conditions which interrupt Ti diffusion, to support our interpretation. The temperature induced modulation of the Au-TMD interface will degrade device performance, and requires to adapt the current strategies for device integration.

Being a very hard anion, oxygen bonds very differently to transition metals than do the chalcogens. Trace oxygen in a transition metal dichalcogenide (TMD) has a substantial impact on material processing and properties, much more so than for instance trace selenium in a sulfide. Since oxygen is all around us, even in our high-vacuum chambers, it is essential to understand and control the effects of oxygen on processing 2D materials. We report the results of three studies of the effect of oxygen on processing TMDs. We find that lowering trace oxygen concentration in the reactor makes it possible to lower the processing temperature for large-area TiS₂ films, made by reacting Ti thin films with H₂S gas. We quantify how lowering oxygen concentration enables faster metal sulfurization at lower temperatures (down to 500°C), leading to thin films that are smooth and homogeneous. In contrast, we find the opposite trend for MoS₂: adding trace oxygen enables lower processing temperatures (down to 375°C) for large-area MoS₂ films, made by sulfurizing Mo thin films. We understand these contrasting effects in the light of particulars of Ti-O and Mo-O bonds, including molecular dynamics (MD) simulations that suggest that oxygen is a catalyst for Mo-S bond formation. We also report a quantitative study of the rate of oxidation of freshly-cleaved surfaces of MoS₂ and Zr(S,Se)₂. MoS₂ surfaces remain pristine for over a year in laboratory ambient conditions, without a trace of oxide formation. In contrast, Zr(S,Se)₂ alloys oxidize rapidly, with the native oxide growing at a rate up to 0.5 Å/min. MD simulations reveal the kinetic mechanisms that limit native oxide growth for MoS₂ and promote it for Zr(S,Se)₂, despite oxide formation in ambient conditions being thermodynamically-favorable in all cases.

Considering their widespread use in future electronics and photonic applications, 2D materials are attracting the attention of the metrology community.[1] Considered an exciting class of 2D materials, transition metal dichalcogenides (TMDs) can be used to build devices whose optoelectronic properties are strongly influenced by the intrinsic and extrinsic defects, growth and transfer methods involved in their fabrication. Furthermore, considering complex integration schemes and stacked TMDs heterostructures, the detailed comprehension of the interaction of defects between layers and the capability to sense them have an even higher importance.[2]Therefore, it is crucial to develop metrology solutions to interpret defects-related phenomena on the nanometer scale for TMDs, and this remains a challenge for the community. Notwithstanding the complexity, the well know flexibility of atomic force microscopy (AFM) offer multiple alternatives for sensing TMDs properties in individual layers and heterostructures. For example, using electrical and electrostatic tip-sample interactions, as in conductive atomic force microscopy (C-AFM) and Kelvin probe force microscopy (KPFM), various atomic-scale defects can be identified and characterized.[3] Similarly, near-field methods such as tip-enhanced Raman spectroscopy (TERS) and compositional techniques such as secondary ions mass spectroscopy (SIMS), can team up to obtain the direct correlation between growing substrates and TMDs films quality. Here, using these and other techniques, we present a metrological analysis scheme that allow the (averaged) electrical characteristics of TMDs-devices to be correlated to the local material properties for a deeper connection to device physics.

References

[1] D. Akinwande, et al., Nature, 2019, 573, 507–518.

[2] H. Lin, et al., arXiv:1611.06457, (2016).

[3] J. Ludwig, et al., Nanotechnology, 2019, 30, 285705.

Van der Waals interactions play a major role in the science of monolayer materials, yet their measurement has received a limited attention. We exploit AFM to measure an attractive van der Waals (vdW) force acting on a sharp AFM tip from a composite sample consisting of a monolayer material supported on a thick substrate. The force is measured as a function of a separation between the tip and the monolayer/substrate stack in the range from 2-20 nm for graphene/silicon oxide, fluorinated graphene/silicon oxide, MoS₂/graphite and MoSe₂/graphite. The obtained results indicate that distinct contributions to the force from the monolayer and substrate can be distinguished by their different dependence on the separation, an inverse cubed for the former and inverse square for the latter. Thus, van der Waals interaction for different monolayer materials is determined and compared to the traditional bulk materials. Further, we demonstrate that the monolayer materials screen van der Waals interactions of the underlying substrate, with full screening in graphene/silicon oxide and partial screening in fluorinated graphene/silicon oxide, MoS₂/graphite and MoSe₂/graphite.

Many interesting material properties cannot be described without a many body electronic structure approach. Electrons may interact not just with other electrons but also couple to bosons, for instance by electron-phonon coupling (EPC). ARPES measurements have revealed ‘kinks’ in the electron dispersion curves and EPC has been discussed as one possible origin [1-4]. This interaction highlights the need of phonon dispersion data which today largely relies on simulations. An experimental technique to access surface phonon dispersion is high resolution electron energy loss spectroscopy (HREELS). Traditionally, HREELS involved mechanically rotating the setup probing one angle and energy channel at the time. Operating such a setup is time consuming and has limited angular resolution. Here, we present a HREELS solution with monochromatic collimated electron source and a hemispherical analyzer [5]. The 2D detector of the analyzer, which simultaneously spans hundreds of channels in both the energy and angular directions, enables a massive speed-up in data collection. While ARPES enables electron dispersion measurements, HREELS can complement these with surface phonon dispersion curves in the same instrument.

[1] Lanzara, A., Bogdanov, P., Zhou, X. et al., Nature 412, 510–514 (2001).

[2] Yang S.-L., Sobota J. A., Shen Z.-X. et al., Phys. Rev. Lett. 114, 247001 (2015).

[3] Akebi S., Kondo T., Shin S. et al., Phys. Rev. B 99, 081108 (2019)

[4] Zhengwang C., Minghu P. [https://aip.scitation.org/author/Pan%2C+Minghu] et al., APL Materials 7, 051105 (2019)

[5] Ibach H., Bocquet F. C., Tautz F. S. et al., Review of Scientific Instruments 88, 033903 (2017)

High conformal with large area transition metal dichalcogenides (TMDs) crystalline thin films were grown at optimum substrate temperature using physical vapor deposition. Fundamental films properties were investigated using Structural, morphological and electrical techniques. XRD data revealed that TMDs (MoS₂ and WS₂) films are highly crystalline in nature and oriented in 00l plane. Thermal conductivity behavior of MoS₂ film also shows low value at room temperature which is excellent to achieve higher thermoelectric figure of merit close to 2. Raman spectroscopy data shows two distinct MoS₂ vibrational modes corresponding to signature E¹_2g and A¹_g peak of TMDs. Electrical, thermoelectric transport studies further demonstrated that MoS₂ films show p-type thermoelectric characteristics, while WS₂ is an n-type material. Finally, highly-efficient thermal energy harvesting pn-junction based thermoelectric generator were fabricate out of these TMDs for waste heat recovery and converted to electrical energy.

A necessary tool for reducing the environmental impact of electronic devices is developing higher-performance and more energy-efficient components. Stacked heterostructure devices of two-dimensional (2D) materials are emerging as promising building blocks for such applications [1]. In such devices, conductive, semiconductive, and dielectric 2D materials are combined to reach the desired electrical function. The most used dielectric in 2D applications is hexagonal boron nitride (h-BN). However, direct synthesis of h-BN on top of 2D materials is complex due to their inert nature and the low growth temperature requirements for stacked electronics.

An exciting solution for these manufacturing constraints is to substitute the crystalline 2D h-BN with a thin film of amorphous-BN (a-BN) [2]. It has been recently reported that ultrathin films of a-BN have a bandgap similar to their crystalline counterpart and retain much of the valued chemical inertness and high thermal stability of h-BN [3]. Moreover, a-BN can be easily manufactured with high spatial resolution and low-temperature processing using Focused Electron Beam Induced Deposition (FEIBD) [4]. However, even though FEIBD is a widely used method [5], a thorough understanding of the electron assisted deposition, where multiple simultaneous processes compete with each other, is lacking.

In this contribution, we report on our surface science study of electron beam-induced deposition of amorphous-BN_x on graphene at room temperature [6]. Using a combination of X-ray Photoelectron Spectroscopy, Low Energy Electron Microscopy, and Scanning Tunneling Microscopy, we identify the essential steps of the deposition process, which highly influence the morphology of the heterostructure, its chemical composition, and its interaction with the substrate. We reveal a much higher decomposition of the precursor molecule than previously thought and a strong interaction with the substrate, which does not affect the structural integrity of the graphene. These results help explain the high boron concentration usually found in FEBID BN films and the thermal stability of the resulting structure.

Our studies provide a valuable surface science insight into using electron beams to synthesize stable and inert layers in 2D heterostructures. Moreover, we aim to spark a discussion about further applications of FEBID, such as confined doping and functionalization of graphene (or other 2D films), patterned adsorption of mass-selected ions, or patterning of adsorbates as anchors for attaching bigger molecules.

[1] Novoselov, K. S., et.al., Science 2016, 353

[2] Glavin, N. R., et.al., Adv. Funct. Mater. 2016, 26.

[3]Hong, S., et.al., Nature 2020, 582.

[4] Sprenger, et.al., J. Phys. Chem. C 2018, 122.

[5] Barth, S., et.al., J. Mater. Chem. C 2020, 8.

[6] Boix, V., et. al., Appl. Surf. Sci. 2021, 557.

Low-dimensional materials display novel emergent properties at the forefront of solid-state physics and chemistry and thus are of both fundamental scientific and applied technological interest. Here, van der Waals (VDW) materials at the 2-D limit, that is transition metal silicates in "quasi-bilayer" phases, supported on a different metal surface (e.g., epitaxial metal film¹). The electronic and elastic coupling of the oxide nanophase to the underlying metal support creates a hybrid system with novel physical and chemical properties that are not shared by the individual constituents. I will investigate two topics in this talk: i) a 2-D oxide phase, which we have prepared by a solid state chemical reaction in two dimensions; and ii) a post-growth hydroxylation study of Fe-silicates on Pd(111) and Au(111) surfaces by using H₂O, H₂, and O₂ as probe molecules, both to gauge the electronic and chemical properties of the M-silicates systems and to investigate its functioning as multiferroic materials in the future. In the first part of the talk, I will concentrate on new structure concepts and their relation to the electronic and magnetic behavior of 2-D systems. In the case of the Fe-silicates, I will introduce a new approach to fabricate well-ordered oxide systems in low dimensions. The computational studies further reveal that the M-silicates (M= Fe, Co, Ni, Mn)can be stable, and further that the transition metal oxidation state may be tuned through the M:Si ratio and by altering the hydroxylation of the layer.²

[1] Hutchings, G. S.; Jhang, J. H.; Zhou, C.; Hynek, D.; Schwarz, U. D.; Altman, E. I. ACS Appl. Mater. Interfaces 2017, 9 (12), 11266–11271.

[2] Saritas, K.; Doudin, N.; Altman, E.; Ismail-Beigi, S.submitted to Physical Review Materials.

Transition metal dichalcogenides (TMDs) exist in several polymorphs including 2H (usually semiconducting), and 1T/1T’ (usually semi-metallic). Our theoretical calculations and experimental measurements show that TMDs are optically-dense and can feature a difference in refractive index greater than unity between phases in the near-infrared (NIR). Fast, non-thermal martensitic phase-change behavior has been observed in TMDs, and transformation strains are expected to be low due to the layered crystal structure. For these reasons, we suggest that TMDs may be useful as active materials to control the phase of light in integrated photonic circuits, surpassing the performance of traditional phase-change materials.

Sulfide TMDs are attractive for application to photonics because they present lower optical loss and higher material stability than selenides or tellurides. However, sulfide TMDs also have higher transformation energy barriers. We address this problem by designing sulfide TMD alloys that are thermodynamically-adjacent to phase boundaries. We use density-functional theory (DFT) and the quasi-harmonic approximation to calculate free energy-composition diagrams at finite temperature for polymorphs in the MoS₂-TiS₂-ZrS₂ system. Our calculations predict that the free energy difference between phases can be reduced to near-zero through alloy design. We then synthesize sulfide TMD thin films through a two-step process of metal sputtering, followed by sulfurization in H₂S in a chemical vapor deposition (CVD) furnace. This two-step method, combined with combinatorial sputtering, enables rapid exploration of composition space and phase boundaries. By controlling the CVD furnace conditions, and in particular the trace oxygen concentration, we are able to lower the processing temperature for large-area alloy thin films below 500 °C; this is important for yielding good film morphology and for photonic integrated circuit integration. Finally, we demonstrate a test concept to study phase-change behavior with electrical stimulation and optical readout for a composition spread across a wafer, and we characterize the phase-change behavior of sulfide TMD alloys by Raman spectroscopy, spectroscopic ellipsometry, and infrared spectroscopy.

[1] A. Singh, S. S. Jo, Y. Li, C. Wu, M. Li, and R. Jaramillo, Refractive Uses of Layered and Two-Dimensional Materials for Integrated Photonics, ACS Photonics 7, 3270 (2020).

Layered van der Waals (vdW) transition metal dichalcogenides (TMDCs) with the chemical formula MX₂ (M = transition metal from Groups 4-7 and X = S, Se, Te) are a family of widely studied materials that have expanded the range of electronic and electrochemical properties associated with two-dimensional (2D) materials, and their potential technological applications. In this work, we use density functional theory calculations for monolayer TMDCs with multiple cation species to point out the exciting possibility to further expand the range of 2D materials (and their properties) by showing that TMDCs with as many as five cation species, dubbed high-entropy TMDC (HETMDC) can be stable in either the 2H or 1T phases at temperatures between 100 K and 500 K. We have used random alloys of five cations (out of a total of nine cations) and showed that over 200 high-entropy TMDC alloy monolayers are likely to be realized experimentally in 2H or 1T phases, and also have shown that the stacking of such monolayers remains governed by vdW interactions. We have evaluated the sulfur vacancy formation energies in multication alloys and estimated the vacancy populations for a wide range of temperatures. We have found that HETMDC alloys display significantly larger vacancy populations compared to simple TMDCs at the same temperatures, due to a very wide range of vacancy formation: these results imply that the catalytic activity of HETMDC is vastly superior to TMDCs, hence illustrate the possibility of high-activity catalysts based on HETMDCs.

It has recently been demonstrated that the angle between layers of two-dimensional materials can strongly impact the resulting properties, inspiring the rapidly developing research area of twistronics. Here, we investigate stacking-dependent optical properties in bilayer WSe₂. Both 2H and 3R stacking orientations are synthesized by chemical vapor deposition. Samples are investigated using photoluminescence, Raman spectroscopy, and reflectivity measurements under ambient and cryogenic conditions. In both 2H and 3R systems, the A_1g Raman mode is sensitive to excitation conditions, with orders of magnitude enhancement observed for certain excitation wavelengths. However, the laser wavelength leading to maximum enhancement is distinctly different for the two stacking orientations, with 2H-WSe₂ exhibiting maximum enhancement under 514 nm excitation and 3R-WSe₂ at 520 nm excitation at cryogenic temperatures. DFT calculations and reflectivity measurements indicate differences in band structure between the two systems, evident by shifts in emission energy of excitonic features, and elucidate the source of variation in Raman spectra. Maximum Raman enhancement is achieved when the excitation wavelength is resonant with the stacking-dependent C-excitonic feature. This work provides a comprehensive investigation of optical properties in 2H- and 3R-WSe₂ bilayers.

Exploration of electronic and magnetic properties of 2D materials family is in a good advancement, with the goal of minimizing electronic devices towards atomic level. This family includes a huge range of materials- from insulators, semiconductors, metals to superconductors. In 2017, two independent groups demonstrated stable magnetic ordering in 2D vdW materials (CrI₃ and Cr₂Ge₂Te₆) ^1,2 bringing in the ferromagnetic materials in 2D family. Since then, 2D magnetism has been discovered in various vdW materials, for example CrX₃ (X= Cl, Br, I) ^3,4 among which CrBr₃ is the most air stable in CrX₃ family. Magnetic and optical properties of bulk CrBr₃ have been studied from 1960s. Despite the great attention in magnetic and optical properties, the electronic properties of CrBr₃ are relatively unexplored. Based on the various optical measurements, it is believed that CrBr₃ have an energy gap in the range of 1.68 – 2.1 eV^5,6. Density fluctuation theory (DFT) reports show even higher deviations in the energy gap. These controversial results have indicated that the electronic properties of CrBr₃ is not well explored.

In this talk, I will present the result from STM/S of both thin and thick CrBr₃ flakes along with density fluctuation theory (DFT) calculations to reveal small energy gap to be 0.57 eV ± 0.04 eV⁷. This uncovering of small energy gap will solve the controversy and is the key to better understand the electronic properties of CrBr₃.

1. Huang, B. et al.Nature 546, 270–273 (2017).
2. Gong, C. et al.Nano Lett. 546, 265–269 (2017).
3. Kim, M. et al.Nat. Electron. 2, 457–463 (2019).
4. Chen, W. et al.Science. 366, 983–987 (2019).
5. Wood, D. L. et al.J. Chem. Phys. 39, 890–898 (1963).
6. Dillon, J. F. et al.J. Appl. Phys. 34, 1240–1245 (1963).
7. Baral, D. et al.Phys. Chem. Chem. Phys 23, 3225 (2021).

We present the electronic characterization of a bottom-up solution-synthesized heterostructure made of the 9-A and chevron graphene nanoribbon (GNR) with C₁₀H₂₁ functionalized side chains. Atomically precise graphene nanoribbons with functional side chains have the potential of being connected to DNA origami scaffolding as a method of achieving deterministic GNR length and precursor arrangement. We use the dry-contact transfer (DCT) method to exfoliate the GNRs onto clean Si(100) in a room temperature ultra-high vacuum (UHV) scanning tunneling microscope (STM). The bandgap and density of states are probed using scanning tunneling spectroscopy (STS). We find that there is a semi-transparency effect across the bulk of the GNR most likely due to random C-Si bonds forming at the GNR-substrate interface. There is also an induced doping effect on the substrate next to the GNR which is most pronounced along the top and bottom edges of the GNR.

In the wake of the discovery of graphene, the search for new and remarkable 2D materials with astounding electronic and mechanical properties has led to the fabrication of germanene, a 2D germanium allotrope similar to silicene. Unlike the planar structure of the graphene lattice, germanene has a buckled honeycomb structure with two vertically displaced sublattices. Free-standing germanene is a semimetal, where the electrons behave as massless relativistic particles leading to enhanced carrier mobility. Indeed, recent studies have shown germanene to have an intrinsic carrier mobility on the order of 6x10⁵ cm²V^-1s^-1[1], an order of magnitude greater than graphene’s. Another advantage over graphene is germanene’s larger spin-orbit gap (23 meV)[2], which when compared to graphene’s (<0.05 meV) makes germanene a superior candidate to exhibit the quantum spin Hall effect at experimentally viable temperatures. Lastly, the germanene lattice allows for an opening of the band gap via an applied electric field or adsorption of foreign atoms, enabling the creation of germanene based field-effect devices. In this study we analyze the effect of edge passivating species H, C, N, P, As, O, S, Se, Te, F, Cl, Br, and I on the electronic and spin-orbit properties of germanene armchair and zigzag nanoribbons. For each species, the effect of width and strain on the band structure is examined, as well as the magnetic moment and spin orbit gap of the nanoribbons to explore potential applications for spintronic devices. We found that for each passivating species, the armchair nanoribbons transition between semiconductor and semimetal in a cyclical pattern depending on width. The bandgap of the nanoribbon, as well as the semimetal point, are tunable through width, passivation species, and mechanical strain. In the case of zigzag germanene nanoribbons, the material remains a semimetal regardless of width, but the nature of the band structure varies significantly among different passivation species with some displaying potential superconducting properties. The different passivation species reveal distinct groups showing similarities in both the geometric structure and band structure of the nanoribbons over various widths. Using machine learning we attempt to classify these passivation species into distinct groups that generate nanoribbons with similar physical and electronic properties.

References:

1. X. Ye, Z. Shao, H. Zhao, L. Yang, C. Wang, RSC Adv. 4, 21216-21220 (2014).
   https://doi.org/10.1039/C4RA01802H
2. M. Kurpas, P. Junior, M. Gmitra, J. Fabian, Phys. Rev. B 100, 125422 (2019).
   https://doi.org/10.1103/PhysRevB.100.125422
3. L. Shi, Q. Li, Phys. Chem. Chem. Phys., 23, 3595-3605 (2021).
   https://doi.org/10.1039/D0CP06231F
4. L. Wu, K. Gu, Q. Li, Appl. Surf. Sci. 484, 1208-1213 (2019).
   https://doi.org/10.1016/j.apsusc.2019.04.071

2D tellurene (Te) nanosheets have recently emerged as promising building blocks for nanoscale high-performance electronic and optoelectronic devices due to their unique electrical and optical properties. Here, we report the synthesis of large-area, high-quality 2D Te nanosheets using a facile one-pot hydrothermal approach, and demonstrate for the first time their use for electrochemical detection of hydrogen peroxide (H₂O₂) with micromolar sensitivity. The as-synthesized 2D Te nanosheets were characterized by Raman spectroscopy, scanning electron microscopy, and atomic force microscopy. The electrochemical performance of 2D Te nanosheets was examined by cyclic voltammetry and amperometry. The 2D Te nanosheets-based sensor shows selective detection of H₂O₂ in the presence of higher concentrations of common interfering substances in human sweat, including NaCl, ascorbic acid, uric acid, and dopamine. An exploration of the feasibility of this methodology for glucose detection in artificial human sweat is currently underway. The findings reported here shed new light on the development of a new generation of flexible and wearable glucose sensors based on 2D materials.